1. Field of the Invention
The present invention relates to a method for producing a nickel powder with reduced impurities, particularly carbon and sulfur from nickel powder produced from a nickel solution by a complexing reduction method.
2. Description of the Related Art
Examples of the methods for smelting nickel include: a method of roasting ore into the form of a sulfide or an oxide and reducing the sulfide or the oxide to obtain ferronickel which is an alloy with iron and used as a raw material for stainless steel; and a method of separating impurities from acid-dissolved solution in which a sulfide is dissolved in hydrochloric acid or sulfuric acid and performing electrowinning to obtain electric nickel. Further, a nickel salt such as nickel sulfate and nickel chloride may be recovered from the acid-dissolved solution and used for plating, a battery material, and the like.
In addition, examples of the methods for producing nickel in a powder state from a nickel salt include a wet process shown in “The manufacture and properties of Metal powder produced by the gaseous reduction of aqueous solutions”, Powder metallurgy, No. 1/2 (1958), pp 40-52.
The method of “The manufacture and properties of Metal powder produced by the gaseous reduction of aqueous solutions”, Powder metallurgy, No. 1/2 (1958), pp 40-52 is a so-called complexing reduction method including: mixing a complexing agent with a nickel sulfate aqueous solution to be subjected to complexing treatment to form a nickel ammine complex solution, putting the solution in a pressure vessel, sealing the vessel, heating the solution to about 150 to 250° C. followed by maintaining the temperature, and blowing hydrogen gas into the solution, in which the nickel ammine complex is reduced by hydrogen to produce nickel powder.
Further, when nickel powder is used as a paste and a positive electrode active material of a nickel-hydrogen battery and the like, impurity elements such as carbon and sulfur may cause the generation of gas. Therefore, the reduction of impurity elements is required.
Therefore, in order to remove sulfur and carbon, a method of heat treatment has been proposed. For example, Japanese Patent Laid-Open No. 2012-31446 discloses a method for producing a ferronickel raw material from a nickel sulfide or a mixed sulfide containing nickel and cobalt, obtained by hydrometallurgy of nickel oxide ore or obtained from scraps or products in process.
Specifically, the ferronickel raw material from which sulfur is separated is obtained through the following steps:
(1) a redissolution step, wherein a nickel sulfide or a mixed sulfide of nickel sulfide and cobalt sulfide is made into a slurry, and an oxidizing agent is added to the slurry to obtain a concentrate containing nickel when the nickel sulfide is dissolved, or a concentrate containing nickel and cobalt when the mixed sulfide is dissolved; (2) a deferrization step, wherein an alkali is added to the concentrate obtained in the redissolution step to obtain a neutralized precipitate and a post-neutralization solution; (3) a solvent extraction step, wherein the post-neutralization solution obtained in the deferrization step is mixed with an organic extractant to be separated into an extraction organic and a raffinate, and then a back-extraction solution and a back-extracted organic are obtained from the extraction organic; (4) a hydroxylation step, wherein alkali is added to the raffinate obtained in the solvent extraction step and mixed to form nickel hydroxide; (5) a roasting step, wherein the nickel hydroxide obtained in the hydroxylation step is heated and roasted in a temperature range of not less than 230° C. and not more than 870° C. to form nickel oxide; and (6) a washing and calcining step, wherein the nickel oxide obtained in the roasting step is water-washed with water at a temperature of not less than 50° C., and then calcined at a temperature of not less than 50° C. to form a washed nickel oxide.
However, unlike the method for producing a ferronickel raw material described in Japanese Patent Laid-Open No. 2012-31446 in which impurities such as carbon and sulfur are removed by heat treatment, although impurities such as sulfur and carbon can be removed by heat treatment in the case of nickel powder, even the nickel powder is simultaneously oxidized or sintered to be coarsened. Therefore, nickel powder in a desired form cannot be produced, and the sintering of nickel powder is not preferred in terms of cost since new facilities for crushing and the like are required.
Thus, a method suitable for effectively separating sulfur and carbon from nickel while avoiding the influence on the properties of nickel powder has not been found.
In order to improve the quality of nickel powder produced by a complexing reduction method, the present invention provides a production method for reducing the content level of sulfur and carbon which are impurities in nickel powder.